Sven Zacek / Wood sorrel (Oxalis acetosella) flowers, Southern Estonia.

Wild Wonders of Europe / López / Leaves of various plants: Lily-of-the-valley, (Convallaria majalis) (top left corner)Wood-sorrel (Oxalis acetosella) with trefoil leaves and Woodruff (Galium odoratum) with leaves in whorls, growing a...

Sven Zacek / Wood sorrel (Oxalis acetosella) flowering on forest floor of boreal forest. Tartumaa, Southern Estonia. May.

Sven Zacek / Wood sorrel (Oxalis acetosella) flowering, soft focus. Boreal forest, Otepaa Nature Park, Valgamaa, Southern Estonia. May.

SCOTLAND: The Big Picture / Wood sorrel (Oxalis acetosella) leaves at base of tree in ancient Ash (Fraxinus excelsior) woodland in spring. Rassal Ashwood National Nature Reserve, Highlands, Scotland, UK.

Andy Sands / Wood sorrel (Oxalis acetosella) two flowers after a rain shower, Suffolk, England, UK, April

Konrad Wothe / Common wood sorrel (Oxalis acetosella) and fern, Bavarian Forest National Park, Germany, May.

Robert Thompson / Wood sorrel (Oxalis acetosella) Co. Armagh, Northern Ireland.

Alex Hyde / Wood sorrel (Oxalis acetosella) growing on woodland floor. Plitvice Lakes National Park, Croatia. November.

Fabrice Cahez / Common wood sorrel (Oxalis acetosella) leaves, Vosges, France, May.

David Tipling / Common wood sorrel (Oxalis acetosella) and Bluebells on woodland floor. Foxley Wood National Nature Reserve, Norfolk, UK, April 2014.

Tony Phelps / Grand Duchess sorrel (Oxalis pes-caprae) and Hedgehog lily (Massonia sp.) De Hoop Nature Reserve, Western Cape, South Africa.